Wherever shafts rotate in housings one finds bearings of one type or another to support such shafts. Single row tapered roller bearings organized in pairs and mounted in opposition perhaps function best in an installation of this type. These bearings carry heavy radial loads and thrust or axial loads as well. Being mounted in opposition, they can be adjusted against each other to varying conditions of end play or preload. Too much end play concentrates loads in limited regions of the bearings and furthermore creates radial and axial clearances which detract from the stability of the shaft. Preload insures that the shaft will rotate about a fixed axis and thus provides stability, but too much preload increases friction within the bearings and may lead to early bearing failure. Thus, the adjustment of oppositely mounted bearings against each other to achieve the proper setting represents an important step in the assembly of a shaft in a housing.
The typical tapered roller bearing has an inner race or cone, an outer race or cup and a single row of tapered rollers located between raceways on the cone and cup. When two such bearings are mounted in opposition, either the small ends of the rollers in the two rows are presented toward each other (indirect mounting) or the large ends are presented toward each other (direct mounting). Irrespective of the arrangement, the bearings are adjusted by controlling the spacing between the cups of the two bearings or the spacing between the cones. Installing shims or spacers behind a cup or a cone in a bearing arrangement represents one of the more common practices for controlling the spacing that determines the setting for the bearings.
Perhaps trial and error is the least complex procedure for determining the size of a shim or spacer, but this procedure lacks precision and is more time consuming. More sophisticated procedures rely on measurements. Indeed, one procedure, in effect, projects surfaces out of a bearing where the distance between the surfaces is easily measured. From these measurements one can select a shim that will provide a setting that closely proximates the desired setting.
But in the typical bearing arrangement having two single row tapered roller bearings factors other than the measurements taken from the projected surfaces affect the setting. For example, an interference fit between the cylindrical exterior surface of a cup and the cylindrical housing bore into which the cup fits will shrink the diameter of the cup raceway and thus change the width of the bearing. That in turn will alter the setting of the oppositely mounted bearings. The typical procedure for projecting surfaces takes the interference fit into account only from the standpoint of the nominal dimensions for the interfering cylindrical surfaces that produce the interference fit. But tolerances in the outer diameter of a cup and in the bore into which it fits can vary the magnitude of the interference fit, and this in turn will produce a variance from the desired setting for the bearings. To be sure, the cylindrical surfaces may be measured separately, but the components that make up the setting must be serialized and united at assembly.